Drill sharpening machines and apparatus of the prior art, for sharpening large drills, have been relatively large, limited in drill capacity range, expensive, time consuming and complicated to operate in order to attain precision drill point sharpening.
Consequently, many machinists have resorted to the hand holding of drills while sharpening the facets on the point of a drill. However, such sharpening of drills depends entirely upon the skill of the operator and has been substantially unsatisfactory for some precision drill operations. A properly sharpened twist drill will have cutting lips at each facet of the drill point which are precisely disposed at the same conical angle, as well as precisely disposed relative to each other in an axial direction so that both cutting lips cut evenly so as to uniformly maintain proper axial alignment operation of the drill and to prevent uneven loading of the cutting lips laterally of the axis of the drill, and also to maintain the ability of the drill to perform for long periods of time without requiring the resharpening. Furthermore, a properly sharpened drill does not tend to overhead one cutting lip more than another and therefore, the drill is maintained at a uniform temperature at opposite sides of its central or rotary axis and further, operates at a lower temperature so that it is capable of a greater amount of work within a given period of time. In addition, a properly sharpened drill produces a more accurate hole size.
The precision sharpening of drill points has therefore been limited to rather complicated machines which are relatively complicated to operate and which require a substantial amount of time for setting up a drill to be sharpened. Furthermore, precision drill sharpening machines are very expensive and many machine shops cannot afford to have one. Additionally, it has been well substantiated that other current drill sharpening machines are limited as to the drill diameter size range.
Every twist drill has a least two flutes extending throughout the length thereof. Each such flute has a leading or cutting edge and a trailing edge with a helical groove between these edges. From the practical viewpoint, it is undesirable, if not impossible, to utilize either the cutting edge or the groove as a guide for positioning the drill relative to the grinding wheel. This undesirability is attributed to the fact that the cutting edge is subject to a large amount of wear and occasionally engages hard spots in the work, resulting in the formation of knicks, small recesses and the like in the cutting edge. While the groove is not quite so susceptible of being marred or impaired as the cutting edge, it is of large area and is also unsuitable for use as a guide.
The present invention is founded largely on the fact that the trailing edge of the flute is the ideal guide for positioning the drill during sharpening because it does not become impaired or damaged during drillig operations. This is due to the trailing edge being relieved, therefore providing providing clearance between flute surface and the cylindrical walls of the hole in the parent metal. Thus, there is no wear or distortion imposed on said trailing edge of the flute.